Duplicate mesh section and merge
In my opinion your method is too tedious and your topology has too many vertices and ngons. What you should do in your case is:


Create a module like this one.





Enable the Snap tool (or press Ctrl) in vertices mode and enable the Auto-merge option.





Duplicate the module as much as you want, move on the X or Y axis (GX and GY), snap and auto-merge will help to make them stick each-other. Give it a Subdivision Surface modifier (and Shade Smooth) to make it look smoother.